Method of removing coatings from metallic conductors



' conductorsto allow of satlsf presents' appreciable difllcuities: are available and have hitherto cant using inorganic acids,

- the use of fused inorganic/salts the cases of a forgot the degreerof success was met, yet in theimain, and

. by reason of the excess Patented June 3,

um'rso "s r r Phyllis Holmes,

as] PA'rE METHOD or nnmovmc. comm METALLIC connnc'rons GS FRQM :Stretford, England, assignmto Metropolitan-Vickers Electrical Company Limited,acompany of Great Britain 7 larly to the removal of electrically insulatingenamel coverings from electric conductors. I The removal of insulating enamel from electric actory connection, as by soldering,to other conductors, generally Twojmethods the one involving the'mechanical scrapingof the covered wire or'other conductor, the dissolving of the enamelby organic solvents.

In the case of relatively thick conductors, ap-

preciable time is required to effect mechanical scraping, while in the case of fine wires such as used in instrument work,-the'time factcriis still more serious and even the most careful handling may result in'mechanical damage to the wire; these difliculties are further enhanced in the case of stranded conductors especially where individual strands are to be connected to corresponding strands. of another conductor, such as commonly used in radio frequency work, Where removal of the enamel covering is eflected by theuse of organic solvents, the, efiicacy depends on the type and constitution of enamel and the def gree of baking or stoving to which it has been subjected. Moreover,

many of the insulating enamels which have more recently'entered the market are substantially insoluble in the solvents and reagents hitherto commonly employed.

Experiments carried out by the present applisuch for instance as sulfuric acid, for the removal of enamel have been found to be not entirely satisfactory, largely due to the time factor involved and the relatively dangerous nature ofthe acid solvent. I

with relatively. low melting point up to about 600 C. was also found to afford litle success to the solution of the problem. Asregardsinorganic salts; with melting point of from 600 C. upwards, though'in many salts tried, some with one outstanding exception; theyywere objectionable'fromone aspect'or another, such as ively'high melting point of the salt, or the resulting contamination; for example 'oxidation;-' of the metal conductor, or the possibility. of subsequent contamination due to the presenceZZof-IesiduaI traces of thesalt or its'constituents, or the absence of a sound, clean boundary between the conductor-and the part of the coating to be left on the conductor. I s

The present invention is based on the discovery to the' removal or coat i I been practiced,

Likewiser I a cl ims. Cl. 134-38).

. problemfand the determination of the conditions i I part is immersedin a bath of. molten potassium"; d n a t em va of the-im ersed part and thecther Application. December ,s,"i' 19.4's, 512,843. In Great Britain February I selection of the one material which provides the outstandingly satisfactory solution to the in which this material is to be used.

' According to-the present invention, in order to l remove, electrically insulating enamel coating from a copper or like conductor, the enamjelled from the bath, the conductor is bared-by a simple wiping operatiom While potassium iodide is neutra -l in reaction, and ac'c'ordingly not'likely'to cause chemical 'react'ionwith'the'metal of the conductor, it is none-,the-less'an electrolyte, and I accordingly care should be taken-in wipingto e remove any residual potassium iodidefr'om the ,bared conducton It has been foundthat'a simple rub between the fingers has been sufilcient com- I pletely to remove varnish andre'sidual saltgleaving a brightchemically clean surface of copper orlike conductor. c I f I In view of the fact that potassium iodide,

although stable at temperatures intheregion of' its melting point, which is namely 680, C., never- V theless is liable ewhen heatedto appreciably higher temperatures,- to be decomposed with the in practicing the invention, that the hathof potassiur'n iodide" should be maintained at'a temperature only' slightly -alcove its melting point I "but yet sufliciently high to produce a-liquidof ferred togrind the commercial potassium iodide I conveniently water-likjihidity, namely a temperature within to'-'750C.--

, In its usual commercialform, potassium iodide v is liable to decrepitate somewhat'violeintly when heated. Thus, in preparing the bath, it is preand, after'fusing thegroundmaterial at a temperature of about. 700 C., to regrind the fused material before charging into theipot or other receptacle, for the bath, the charge being then I I heated-to,*and maintainediat a temperature of The, time period during which the coated een-- I .ductor should be'left in thebath'willdeipend gen:-

; erally'upon, the gauge ofthe conductor'andfthe thickness of thecoating. An excessivedipping: 1

time duration may lead to slight emhrittlement,

I which is at present thought to be hydrogen em- C for a period of up to oneor brittlement. As a general criterion, wires of, say, 0.03 inch diameter would two seconds for en- 'suring the complete decomposition of the enamel,

of iodine it isimportant.

the range of," say, frcm '700 C.- I

require to be'sdippedcopper wire treated according to the invention a, bright chemically clean surface of copper is left which is ready for soldering without further treatment.

In practicing the invention it is preferred to use a bath of potassium iodide prepared as described, in an electrically heated and thermostatically controlled pot or other receptacle.

While the invention is eminently successful as applied to the baring of copper wires coated with the insulating enamel containing resinous compositions of the polyvinyl acetal type and sold under the trade name Formex," which, to all intents and purposes, is insoluble in all the solvents and reagents hitherto commercially used for the removal of enamel coating, the invention is also applicable for the baring of conductors having any of the various enamel insulating coatings in present-day use.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of removing an electrically insulating coating comprising a polyvinyl acetal resin from an electrical conductor which com- 4' 4 prises immersing the coated. conductor in a bat of molten potassium iodide maintained at a temperature between 700 C. to 750 C. and for a time. suiflcient to effect removal of. the coatin from the conductor and removing any excess potassium iodide remaining on the conductor afterthe immersion.

2. The method of removing an electrically insulating coating comprising a polyvinyl acetal resin from a metallic conductor which comprises subjecting the coated conductor to the action of molten potassium iodide.

3. The method of removing an electrically insulating coating comprising a polyvinyl acetal resin from a copper conductor which comprises immersing the coated part from which the coating is to be removed in molten potassium iodide,

and removing any excess potassium iodide remaining on the conductor after the immersion. PHYLLIS HOLMES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,714,879- Lang May 28, 1929 FOREIGN PATENTS Number Country Date Great Britain Jan. 21, 1932 

